Java 8 stream s min and max why does this compile

Question

Note  this question originates from a dead link which was a previous SO question  but here goes     See this code  note  I do know that this code won t  work  and that Integer  compare should be used -- I just extracted it from the linked question    final ArrayList  lt Integer gt  list        IntStream rangeClosed 1  20  boxed   collect Collectors toList      System out println list stream   max Integer  max  get     System out println list stream   min Integer  min  get       According to the javadoc of  min   and  max    the argument of both should be a Comparator  Yet here the method references are to static methods of the Integer class   So  why does this compile at all

User · Answer

I had an error with an array getting the max and the min so my solution was   int max   Arrays stream arrayWithInts  max   getAsInt    int min   Arrays stream arrayWithInts  min   getAsInt

User · Answer

Apart from the information given by David M  Lloyd one could add that the mechanism that allows this is called target typing   The idea is that the type the compiler assigns to a lambda expressions or a method references does not depend only on the expression itself  but also on where it is used   The target of an expression is the variable to which its result is assigned or the parameter to which its result is passed   Lambda expressions and method references are assigned a type which matches the type of their target  if such a type can be found   See the Type Inference section in the Java Tutorial for more information

User · Answer

This works because Integer  min resolves to an implementation of the Comparator lt Integer gt  interface   The method reference of Integer  min resolves to Integer min int a  int b   resolved to IntBinaryOperator  and presumably autoboxing occurs somewhere making it a BinaryOperator lt Integer gt    And the min   resp max   methods of the Stream lt Integer gt  ask the Comparator lt Integer gt  interface to be implemented  Now this resolves to the single method Integer compareTo Integer o1  Integer o2   Which is of type BinaryOperator lt Integer gt    And thus the magic has happened as both methods are a BinaryOperator lt Integer gt

User · Answer

Comparator is a functional interface  and Integer  max complies with that interface  after autoboxing unboxing is taken into consideration   It takes two int values and returns an int - just as you d expect a Comparator lt Integer gt  to  again  squinting to ignore the Integer int difference    However  I wouldn t expect it to do the right thing  given that Integer max doesn t comply with the semantics of Comparator compare  And indeed it doesn t really work in general  For example  make one small change   for  int i   1  i  lt   20  i        list add -i         and now the max value is -20 and the min value is -1   Instead  both calls should use Integer  compare   System out println list stream   max Integer  compare  get     System out println list stream   min Integer  compare  get

User · Answer

Let me explain what is happening here  because it isn t obvious   First  Stream max   accepts an instance of Comparator so that items in the stream can be compared against each other to find the minimum or maximum  in some optimal order that you don t need to worry too much about   So the question is  of course  why is Integer  max accepted   After all it s not a comparator   The answer is in the way that the new lambda functionality works in Java 8   It relies on a concept which is informally known as  single abstract method  interfaces  or  SAM  interfaces   The idea is that any interface with one abstract method can be automatically implemented by any lambda - or method reference - whose method signature is a match for the one method on the interface   So examining the Comparator interface  simple version    public Comparator lt T gt        T compare T o1  T o2       If a method is looking for a Comparator lt Integer gt   then it s essentially looking for this signature   int xxx Integer o1  Integer o2     I use  xxx  because the method name is not used for matching purposes   Therefore  both Integer min int a  int b  and Integer max int a  int b  are close enough that autoboxing will allow this to appear as a Comparator lt Integer gt  in a method context

[java] Java 8 stream's .min() and .max(): why does this compile?

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